High molecular weight PET can be produced by the reaction of dimethylterephthalate and ethylene glycol, to form the bis glycol ester of terephthalic acid, followed by the catalytic polymerization of this ester to high molecular weight product. It is conventional practice in the latter process to employ a divalent manganese catalyst to affect the reaction between the dimethylterephthalate and ethylene glycol. See Berkau, et al. U.S. Pat. No. 3,551,386, and Busot U.S. Pat. No. 3,487,049.
Cramer U.S. Pat. No. 3,070,575 shows in example II the addition of terephthalic acid (TPA) to a mixture of bis glycol ester of terephthalic acid and manganese catalyst. The amount of terephthalic acid added is over 1000 time the stoichiometric amount of manganese.
It is also known that this divalent catalyst if it remains unchanged in the final product, tends to color the product, and/or make it difficult to obtain a high molecular weight polymer. It is known to add phosphoric acid or other phosphorous compounds to the bis ester prior to polymerization in order to mitigate the color problem and the molecular weight problem.